1. Field of the Invention
The present invention relates generally to fuel cell devices and more particularly to solid oxide fuel cell devices that can minimize device failure due to thermal mechanical stress.
2. Technical Background
Solid oxide fuel cells (SOFC) have been the subject of considerable research in recent years. Solid oxide fuel cells convert the chemical energy of a fuel, such as hydrogen and/or hydrocarbons, into electricity via electro-chemical oxidation of the fuel at temperatures, for example, of about 700 to about 1000° C. A typical SOFC comprises a negatively charged oxygen-ion conducting electrolyte sandwiched between a cathode layer and an anode layer. Molecular oxygen is reduced at the cathode and incorporated in the electrolyte, wherein oxygen ions are transported through the electrolyte to react with, for example, hydrogen at the anode to form water.
Some designs include electrode-electrolyte structures comprising a solid electrolyte sheet incorporating a plurality of positive and negative electrodes bonded to opposite sides of a thin flexible inorganic electrolyte sheet.
SOFC devices are typically subjected to large thermal-mechanical stresses due to the high operating temperatures and rapid temperature cycling of the device. Such stresses can result in deformation of device components and can adversely impact the operational reliability and lifetime of SOFC devices.
The electrolyte sheet of a SOFC device is typically sealed to a frame support structure in order to keep fuel and oxidant gases separate. SOFC devices endure thermal cycling and large thermal gradients, which induces thermal stresses in the electrolyte sheets. In addition, a mounted electrolyte sheet will expand at a rate that is different from the thermal expansion rate of its frame, which may cause cracking of the electrolyte sheet. In some cases, the thermal mechanical stress and resulting deformation may be concentrated at the interface between the seal and the metal frames, resulting in a failure of the seal, the electrolyte sheet, and/or the SOFC device. When a thin, flexible ceramic sheet is utilized as the electrolyte in a SOFC device, there is a higher likelihood of premature failure of the electrolyte sheet itself. Differential gas pressure and interactions between the device, the seal, and the frame due to temperature gradients and the mismatch of component properties (e.g., expansion and rigidity) may lead to increased stress at the seal and the unsupported region of the electrolyte sheet adjacent to the seal. Large electrolyte sheets are especially subject to failure caused fracturing of the electrolyte sheet due to operational or transient stresses.
U.S. Patent Publication 2006/0003213 describes the problem of stress related cracking of the SOFC device electrolyte sheet and discloses a patterned electrolyte sheet designed to compensate for environmentally induced strain and provide increased failure resistance to the device. U.S. Patent Publications 2003/0215689 and 2003/0224238 describe a metal foam seal and a high temperature felt sealing material that can be utilized to address the build up of strain at the bonding region of the electrolyte, seal, and frame. However, alternative and/or additional thermal stress minimization approaches may also serve as mitigation schemes to overcome thermal mechanical failures of fuel cell devices.
Thus, there is a need to address the thermal mechanical integrity of solid oxide fuel cell seals and electrolyte sheets, and other shortcomings associated with solid oxide fuel cells and methods for fabricating and operating solid oxide fuel cells. These needs and other needs are satisfied by the articles, devices and methods of the present invention.